Method of coating metal



Patented Jan. 12, 1937 STATES PATENT OFFICE METHOD OF COATING METAL No Drawing. Application June 24, 1933, Serial No. 677,458

7 Claims. (01. 148-6) This invention relates to coating metal and more particularly it relates to an improvement in methods by which a substantially integral coating is produced by chemical means upon a metal surface which is thereafter covered with a coat of paint, varnish, enamel or the like which dries or hardens over the chemical coat and is hereinafter referred to for brevity as a siccative coat.

It is known that various treatments will produce a substantially integral coat upon metal by chemical changes in the surface of the metal which result in firmer adherence of a siccative coat thereto than can be-obtained by direct application of the siccative coat upon bare metal. This has been practiced to a considerable extent,

particularly with iron and steel, by the production on iron and steel of a phosphate coating asa foundation or bond for paint or the like. Similar phosphate coats may be produced upon zinc or magnesium, and by somewhat similar treatments substantially insoluble salts of the metals may be produced by treatment with organic acidsincluding the dicarboxylic and hydroxy-dicarboxylic acids of the aliphatic series and carboxylic acids, having but one carboxyl group, and sulphonic acids of the aromatic series, particularly where oxidizing agents and copper compounds or the like are employed as accelerators. By proper 40 lithium aluminate or alliminum fluoride. The

nitric acid radical generally facilitates the production of oxide coatings on metal and also the production of integral-salt coats of the kind described above. These various bonding coats are herein- I 45 after referred to for brevity as chemical coats.

While the adherence of siccative coats to the metal is greatly increased by the bonding effect of a chemical coat, it has been found dimcult to avoid blisters between the chemical coat and the' 50 siccative coat, where the chemical coat has been produced with the aid of the N03 radical, particularly when the finished surface is subjected to humidity tests. I g

The object of the present invention is to provide 55 an intermediate treatment after the production of the chemical coat and before the application of the siccative coat, whereby this blistering under the siccative coat is minimized or eliminated.

After the production of the chemical coat, the surface is customarily washed before the applica- 5 tion of the siccative coat. In accordance with the present invention, the chemical coat is preferably rinsed in the usual manner and is then treated with a dilute solution of chromic acid, phosphoric acid or oxalic acid. Treatment with 10 dilute solutions of other acids, with a few exceptions such as nitric acid, reduces to some extent the blistering but not as effectively and satisfactorily as the acids named. A treatment in a dilute solution of nitric acid seems to increase rather 15 than decrease the blistering.

The three acids named above are preferred in the order in which they are named, the chromic acid being decidedly preferable, phosphoric acid being next in efficiency, and oxalic acid being not 20 so desirable but still effective to materially reduce the blistering.

In the preferred form, the chromic acid solution is formed by introducing from 2 grams to 20 grams of chromic acid in a gallon of water. For 25 ordinary commercial purposes about 4 grams of chromic acid to a gallon of water is preferred, but fairly good results may be obtained with a dilution as great as 2 grams in a gallon of water,

and while a strength greater than 4 grams to a gallon of water seems unnecessary, it seems to do little, if any, harm until 20 grams or over are used. Stronger solutions of chromic acid are likely to injure the chemical coat, and even with 20 grams to the gallon undesirable yellow streaks' develop although these streaks do not seem to affect the adherence of the siccative coat.

It is preferable to employ the customary rinsing after the production of the chemical coat and before the use of the chromic acid rinse, so as '40 to avoid contamination of the chromic acid rinse. No treatment is necessary after the chromic acid rinse and before the application of the siccative coating, If any subsequent rinsing. takes place, it should be with distilled water orat least with water which does not contain lime, alkali or other material which may result in contaminating the chemical coat sufficiently to'produce blisters.

The length of treatment in the chromic acid rinse depends upon the strength of the solu tion employed, the condition of the work and the temperature of the rinsing bath. Preferably the bath is employed at a temperature approaching the boiling point and, with the strength 56 the phosphate builds up so that it requires more than four times as much alkali to neutralize a given amount of solution with phenolphthalein as an indicator than it does to neutralize the same amount of solutionwith methyl orange as an indicator. About one minute treatment in such a phosphoric acid rinse is ordinarily suflicient.

Where an oxalic acid rinse is employed, it

should be used hot in approximately corresponding strength to that indicated above for the phosphoric acid solution.

The acid rinse to minimize or eliminate blisters under a siccative coat is particularly effective when employed with a chemical coat produced by a phosphate solution containing nitrates, but is also effective with the other chemical coats mentioned above, which are obtained by means of solutions containing N03 radical. Also, while the most important use of the method at the present time is in connection with coating iron or steel, it is also effective with the chemical coats produced upon other metals, as mentioned above.

What is claimed is:

. l. The method which consists in producing on a metal surface a chemical coating by means of a solution containing the nitric acid radical,-

, rinsing the coated surface; with a substantially pure dilute aqueous solution of an acid in the group consisting of chromic acid, phosphoric acid, and oxalic acid, and thereafter covering said surface with asiccative coat,

2. The method which comprises producing on a metallic surface a phosphate-containing coating by means of a solution containing the N03 radical, rinsing the coated surface in a dilute aqueous solution of chromic acid and covering the rinsed surface with a siccative coat.

3. The method which consists in producing on a surface of iron or steel a phosphate coating by means of a solution containing the nitrate radical, rinsing the coated surface in a dilute aqueous solution of chromic acid, and covering the rinsed surface with a siccative coat.

4. A method in accordance with claim 3, and in which the dilute acid rinse is of chromic acid of the order of strength of 4 grams of chromic acid to one gallon of water.

5. A method in accordance with claim Land in which the acid rinse is a solution of oxalic acid requiring between one-fourth cc. and onehalfcc. tenth normal caustic soda to neutralize 10 cc. of the solution, methyl orange being the indicator.

6. A method in accordance with claim 1, and in which the acid rinse is a solution of phosphoric acid requiring between one-fourth cc. and one-half cc. tenth normal caustic soda to neutralize 10 cc. of the 'solution, methyl orange being the indicator, and requiring not more than four times as much alkali to neutralize a given amount of the solution with phenolphthalein as an indicator than with methyl orange as an indicator.

7.- The method which consists in coating a metal surface of the group consisting of iron, zinc and their alloys, by means of a phosphate solution containing the nitric acid radical, rinsing the coated surface in a phosphoric acid solution requiring not more than one-half cc. tenthnormal caustic soda to neutralize 10 cc. of the solution, methyl orange being the indicator, and not more than four times as much alkali to neutralize the solution with phenolphthalein as' the indicator than withmethyl orange as the indicator. ROBERT R. TANNER. JOHN S. THOMPSON. 

